A wide variety of mobile communication devices, such as cellular phones, use heterojunction bipolar transistors (HBTs) to amplify the power of the radio frequency (RF) signals they transmit to base stations. When an HBT amplifies high-frequency RF signals, its base-collector capacitance may change with collector voltage, and this change can affect the distortion characteristics of the HBT. For better distortion characteristics, this change in base-collector capacitance needs to be small for a given change in collector voltage (i.e., the collector-voltage dependence of base-collector capacitance needs to be low). For example, International Publication No. 2015/005037 discloses an HBT that features a collector layer having a unique doping profile, which makes its base-collector capacitance less dependent on its collector voltage.